Thickness Monitoring for Precise Optical Interference Coatings

摘要

High quality thin film optical filters need high precision film thickness monitoring during the deposition. There are several methods can do the thickness monitoring, named as time-counting monitoring, quartz-oscillation monitoring, traditional optical monitoring, optical admittance loci monitoring and phase-extraction monitoring. Each method has its advantage and disadvantage. Since no information about refractive index of the depositing thin film can be provided by time-counting monitoring or quartz-oscillation monitoring, optical monitoring is preferred if an optical filter needs film thickness controlled very precisely during the deposition, although time-counting monitoring and quartz-oscillation monitoring are cheaper and simpler. Optical admittance loci monitoring has an advantage of visual control so that it has a better error correction mechanism. Besides it can be applied on a traditional optical coating machine by the implantation of software without any additional hardware. Phase-extraction monitoring is the most precise monitoring method, it needs the installation of an interferometer and a camera with polarization array, however, it is vibration insensitive, compact and cheaper than using ellipsometer and having much better performance than an ellipsometric system.